Indeed that is true. Technically it probably isn’t the airline’s responsibility to accommodate both passengers. Then again 9 out of 10 airlines might see that as a better bet than having the bad PR (and I would think Ryanair would be no 10).
The PNR might not explicitly have them as honeymooners but surely it would be clear they were travelling together.
Maybe they got the flight cheap in a sale. Maybe they got it cheap by booking it early as part of a deem holiday?
And OK the ground agents may have been outsourced, however the contract of carriage is with BA, so it’s their job to ensure their suppliers are professional.